The existing manually operated projection screen consists of a pulling tube, a curtain, a torsion spring assembly, a locking mechanism, a rotating tube and a housing, wherein one end of the curtain is connected to the pulling tube and the other end of the curtain is connected to the rotating tube. In use, the curtain is unrolled and locked by pulling the pulling tube at one end of the curtain by hand. When the curtain is no longer in use, the locking mechanism is released, and the curtain is rolled back by the rotating tube driven by the torsion spring assembly so as to return from the unrolled state in use to the state of being rolled back around the rotating tube. A decelerating mechanism is arranged in the rotating tube so that the curtain can be smoothly rolled back. A locking mechanism (i.e. positioning mechanism) is provided so that the curtain can be freely locked in position at any moment.
The existing decelerating mechanism and locking mechanism have the following disadvantages:                1) Arranging the decelerating mechanism and the locking mechanism in the rotating tube makes the structure complex, and makes the assembling inconvenient, thus resulting in high cost;        2) The existing decelerating mechanism for a projection screen has lower transmission ratio. Therefore, when the curtain is rolled back, the speed of the curtain is too fast and thus the pulling tube and the housing collide violently, which may damage the pulling tube and the housing;        3) During the rolling back and the pulling down of the curtain, it is necessary to lock or position the curtain frequently. The existing locking mechanism can lock or position the curtain twice during one cycle of the rotating tube. Assuming the diameter of the rotating tube is 43 mm, the curtain can be locked once when the curtain has moved a distance of 43π/2=67.5 mm. Since the rotating tube can continue rotating a little after being locked, the locking mechanism can not be in a completely locking state after the curtain is completely rolled back. Thus the torsion force of the torsion spring still acts on the rotating tube and is further transferred to the curtain. As a result, the curtain is under a great pulling force for a long time.        
In addition, since the rotating tube can continue rotating a little after being locked, the curtain cannot be rolled around the rotating tube smoothly and stretchedly. In other words, the curtain may be folded partially, thereby some crease or wrinkle may be formed on the curtain because the curtain is under pressure for a long time. Thus, the crease would affect the usability of the projection screen for the next time.